fix: add type test.

Author: mn4367

types/index.test-d.ts

@@ -34,6 +34,12 @@ app.register(fastifyHttpProxy, {
     expectType<RawRequestDefaultExpression>(request.raw)
     expectType<RawReplyDefaultExpression>(reply.raw)
   },
+  preRewrite: (url, params, prefix): string => {
+    expectType<string>(url)
+    expectType<unknown>(params)
+    expectType<string>(prefix)
+    return ''
+  },
   base: 'whatever',
   cacheURLs: 10,
   undici: {